In radio communication systems it is normally necessary for a receiver to determine whether a transmitter is trying to initiate a communication. In such systems where the transmitter is normally active or systems where the air interface includes scheduling information, this issue is readily resolved. However in systems using an asynchronous protocol for communications, determining whether a communication is desired in an efficient manner is still a significant problem. This problem is further exacerbated where the particular frequency for the communications is unknown and by frequency mismatches between the transmitter and receiver, particularly for battery life sensitive applications. The power consumed in monitoring for an asynchronous communication from another device to determine whether an attempt to communicate has occurred may significantly reduce the battery life and thus available service time of the mobile unit.
An asynchronous, dispatch, voice communication system can identify when communications are desired given frequency offsets between communications devices. For example, such asynchronous system can use a preamble signal as a mechanism by which an originating device may “wake up” a target device from a stand-by mode. Transmitting a predetermined waveform to initiate communication between asynchronous devices is known in the art. Such a system operates well from the standpoint of a single user. Unfortunately, such an asynchronous system is not as efficient from a system perspective when multiple users share the same channel. More specifically, if an air interface specifies only one known preamble waveform, then any transmission on a given channel will wake up all receivers from stand-by mode that are within range and monitoring that channel. All of these receivers will then complete the synchronization process with the originating device, processing information related to the call type, target address or ID, etc., to determine if the transmission is intended for them. Using one known preamble disrupts all of these receivers from stand-by mode even though the transmission may not have been intended for them. Such needless disruption of unintended receivers creates a situation where even intended receivers may actually miss transmissions.